1. Field of the Invention
The present invention relates to a coil device preferably used for a resonance transformer and the like.
2. Description of the Related Art
Coil devices are used in various electrical products for various uses. For instance, in a lighting circuit for a backlight of liquid crystal display, a leakage transformer, which is as a resonance transformer for driving a display device with higher voltages, is generally used.
For a leakage transformer, as shown in the following Reference 1 for instance, a horizontal-type coil device, to which a scroll axis of coil is arranged parallel to a mounting substrate surface of the coil device, is known. Such horizontal-type coil device has a problem that a leakage flux toward upward and downward directions with respect to the mounting substrate surface is large.
In order to make the leakage flux small, it is considered that top and bottom of the horizontal-type coil device is covered with aluminum board and aluminum foil. However, with this, heat dissipation may be deteriorated.
Further, for other leakage transformer, as shown in the following Reference 2 for instance, a vertical-type coil device, to which a scroll axis of coil is arranged perpendicular to a mounting substrate surface of the coil device, is known. With its configuration, it enables to make the leakage flux toward upward and downward directions with respect to the mounting substrate surface small.
However, the vertical-type coil device has a problem that a stable operation as a resonance transformer is difficult since a primary coil and a secondary coil are arranged to have the same center and a coupling coefficient K is too favorable (for instance, K=0.95 or more). For instance, in the vertical-type coil device, there is a problem that a separation distance between the primary coil and the secondary coil with the same center has to be subtly adjusted and a core for leakage has to be added to the core, in order to achieve a desired leakage characteristic. Therefore, the conventional vertical-type coil device has a problem that a characteristic fluctuation is likely to be generated since the assembling becomes complicated.    [Patent Literature 1] Japanese Patent Application Publication No. 2006-108390    [Patent Literature 2] Japanese Patent Application Publication No. 2005-158927